diff --git a/local/scripts/build-redbear.sh b/local/scripts/build-redbear.sh
index 0210904730..8f6b6e8dcd 100755
--- a/local/scripts/build-redbear.sh
+++ b/local/scripts/build-redbear.sh
@@ -53,6 +53,48 @@ redbear_is_fork_dirty() {
     return 1
 }
 
+# Per AGENTS.md § "BRANCH AND SUBMODULE POLICY (ABSOLUTE)": each Red Bear
+# fork worktree MUST be checked out on the canonical "submodule/<name>"
+# branch. This gate prevents drift from the canonical branch topology.
+redbear_check_fork_branches() {
+    local err=0
+    local wrong=()
+    for entry in "${REDBEAR_FORK_SOURCES[@]}"; do
+        local label="${entry%%:*}"
+        local dir="${entry#*:}"
+        [ -d "$dir" ] || continue
+        local cur_branch
+        cur_branch=$(git -C "$dir" branch --show-current 2>/dev/null) || continue
+        local expected="submodule/${label}"
+        if [ "$cur_branch" != "$expected" ]; then
+            wrong+=("$label (got '$cur_branch', expected '$expected')")
+            err=1
+        fi
+    done
+    if [ "$err" != "0" ]; then
+        echo "========================================" >&2
+        echo "  REFUSING TO BUILD — FORKS NOT ON CANONICAL BRANCHES" >&2
+        echo "========================================" >&2
+        echo "" >&2
+        echo "Per AGENTS.md § 'BRANCH AND SUBMODULE POLICY', each Red Bear" >&2
+        echo "fork worktree must be checked out on its canonical 'submodule/<name>'" >&2
+        echo "branch. The following forks are not:" >&2
+        echo "" >&2
+        for w in "${wrong[@]}"; do
+            echo "  - $w" >&2
+        done
+        echo "" >&2
+        echo "Fix with (from the parent repo):" >&2
+        echo "  for f in base bootloader installer kernel libredox redoxfs relibc syscall userutils; do" >&2
+        echo "    (cd \"local/sources/\$f\" && git branch -m HEAD submodule/\$f 2>/dev/null || true)" >&2
+        echo "  done" >&2
+        echo "  ./local/scripts/push-fork-branches.sh" >&2
+        echo "" >&2
+        echo "Override (emergency only): REDBEAR_ALLOW_WRONG_BRANCH=1 $0 $*" >&2
+        exit 1
+    fi
+}
+
 redbear_stash_one() {
     local label="$1"
     local dir="$2"
@@ -316,6 +358,12 @@ if [ "${REDBEAR_ALLOW_DIRTY:-0}" != "1" ] && [ -z "${REDBEAR_RELEASE:-}" ]; then
     fi
 fi
 
+# Per AGENTS.md § "BRANCH AND SUBMODULE POLICY (ABSOLUTE)": each Red Bear
+# fork worktree must be on the canonical submodule/<name> branch.
+if [ "${REDBEAR_ALLOW_WRONG_BRANCH:-0}" != "1" ] && [ -z "${REDBEAR_RELEASE:-}" ]; then
+    redbear_check_fork_branches
+fi
+
 if [ -x "$PROJECT_ROOT/local/scripts/verify-overlay-integrity.sh" ] && [ -z "${REDBEAR_RELEASE:-}" ]; then
     echo ">>> Verifying overlay integrity..."
     if ! "$PROJECT_ROOT/local/scripts/verify-overlay-integrity.sh" --quiet; then